Apple Remote Desktop 3.8 Authenticates, but will not connect.

So is the system that is initiating the connection, based on an older version f OS X?

It's probably not much help, but I've broken Observe and Control functionality before, presenting similar symptoms when I unwittingly applied a newer version Remote Desktop Client update on an unsupported client. I tried a number of ways to put the old version back, but in the end I reached a point where reinstall OS X was the remaining option.

I'm having the same problem since upgrading my machine to El Capitan.

If I select two machines and click Observe, I can see them both. If I try to Observe or Control either one individually, it sits on that Connecting.

Some of the machines are 10.10.4 and some are 10.10.5. Some have 3.8.3 and some are 3.8.4 for the ARD version.

I can still connect to my Mac servers (all are older than 10.10).

I can also connect to some, but not all, of my teacher laptops individually.

I talked to an Apple engineer that said he hadn't heard of anything like this, and also suggested reinstalling ARD. That didn't help. I've just kept dealing with it and putting off a wipe/reinstall of the OS.

No, the initiating system is running Mac OS X 10.11.1.

I ran into this issue in the past few weeks and I have a ticket in with AppleCare OS Support to see if there's any way to get this to work.

Do any of you have "12/2/15 8:07:57.491 PM Remote Desktop[44419]: Could not set scaling factor to -0.103704" in your console when you try to connect to a single client?

If you change Preferences > Control & Observe > When viewing a computer with a larger screen to "Show full size" do observe and control now not even show the window for "connecting..."?

Hi Sean, That gave me a temporary workaround for the moment. Thanks! I'll make sure to let AppleCare OS Support know that's a workaround as well. And ask them to work on resolving it ASAP.

Sean, that sounds like it may be the issue. I have been working with multiple monitors and Remote Desktop is always open in the secondary. I also use a MacBook Pro that is never connected to an external monitor and it has not had this issue. I will give it a try and report back. (I won't be able to check until after the holidays.) Thanks!

Thanks Sean. This has been troubling me for a while. Glad to have a workaround.

The issue is not consistent then, or just unique to maybe certain environments? Running OS X 10.11.3 on my 27" iMac ( late 2013).
Using ARD v. 3.8 (380A95) from the Mac App Store.

I have two connected monitors across my desk, to the Mac, one via VGA to TBolt, one DVI to Tbolt.
ARD sits on the second monitor, and I can double Click on any computer in my lists (running 10.9.5, 10.7.5, 10.10.5) and they connect just fine.
(Double Click is set to Control, not observe)

Could the issue simply be a Monitor Settings issue?
I keep all of mine set to Best for Display at 60hz ( except for the iMac).

Just posting my specs which work without issue to the forum, hope it helps others.
Sounds strange that this is occurring, but it isn't widespread AFAIK....

Thanks Sean!!! That worked for me too!!

Thanks for your help Sean!
That worked for me too (monitor + El Capitan 10.11.5 MBP)

Double UU T F!

It worked!

Worked for me also, thank you @seanhansell !